The invention relates to an apparatus for the formation of images of an examination subject with nucelar magnetic resonance, wherein means are present for the application of magnetic fields to the examination subject and for the detection of the deflection of the atomic nuclei of the examination subject from the equilibrium state by means of a high frequency magnetic excitation pulse, whereby the means for the application of magnetic fields includes at least one gradient coil for the generation of a linear magnetic field gradient.
It is known that it is possible to deflect, in particular, the hydrogen atomic nuclei of an examination subject from a preferred direction, which is established by a basic magnetic field, by means of a high frequency excitation pulse, and that the atomic nuclei, after the end of the excitation pulse, due to their spin, swing into the preferred direction again only after a certain time. During this time, the atomic nuclei precess with a frequency which is dependent upon the intensity of the basic magnetic field. If a field gradient is superimposed on a homogenous basic magnetic field so that the magnetic field distribution varies spatially, then a position selection via the respective measured frequency is possible. It is also known that, in this manner and through alteration of the direction of the field gradient, it is possible to prepare tomographic images of the examination subject. The excitation in a layer of the examination subject thus proceeds in that the basic magnetic field is influenced by an additional field gradient such that an excitation of the atomic nuclei takes place only in this layer. This is possible because the excitation proceeds only with the frequency which is strictly associated with the magnetic field in the desired layer.
In the gradient coils for the generation of the field gradient, in practice, coil currents occur in the range of five amperes to 300 amperes which must be capable of being relatively rapidly switched on and off. In order that no image interferences occur, the current pulses in the gradient coils must exhibit a precisely specified shape (flat top) and a reproducible rise and fall as a function of time at the switching transitions; for example, a substantially rectangular waveform as a function of time.
Background concerning the operation of nuclear magnetic resonance equipment is found in an article by Zeitler et al "Nuclear Magnetic Resonance Tomography (NMR Tomography) and Its Clinical Application Possiblities", Electromedica (English Edition) No. 3, 1981, pages 134-143, and in an article by Loeffler and Oppelt "Possibilities and Limitations of NMR Imaging", Electromedica (English Edition), No. 2, 1982, pages 38-40.